1. Field of the Invention
The present invention relates to a current collector for use in an electric double-layer capacitor.
2. Description of the Related Art
As shown in FIG. 6, an electric double-layer capacitor comprises anode electrodes 12 having respective anode current collectors 11 and cathode electrodes 14 having respective cathode current collectors 13. The anode and cathode electrodes 12, 14 in each pair are disposed in confronting relation to each other with a separator 15 interposed therebetween, and jointly make up a basic cell 16. Adjacent basic cells 16 are arranged such that electrodes of like polarity share one current collector. Specifically, the anode electrode 12 of a certain basic cell 16 and the anode electrode 12 of an adjacent basic cell 16 share an anode current collector 11, and the cathode electrode 14 of a certain basic cell 16 and the cathode electrode 14 of an adjacent basic cell 16 share a cathode current collector 13. A plurality of basic cells 16 are stacked into a capacitor assembly 17. On one outermost side of the capacitor assembly 17, there is a separator 15 disposed on an anode current collector 11. On the opposite outermost side of the capacitor assembly 17, there is a separator 15 disposed on a cathode current collector 13.
Each of the anode and cathode electrodes 12, 14 comprises a thin plate of activated carbon. Each of the separators 15 comprises a sheet of fluoroplastic. Each of the anode and cathode current collectors 11, 13 comprises an electrically conductive metal foil which may be of aluminum or the like. Anode leads 18 and cathode leads 19 extend from upper ends of the anode and cathode current collectors 11, 13, respectively. The anode leads 18 and cathode leads 19 are combined and connected to respective electrode terminals. The capacitor assembly 17 is housed in a case (not shown), which is supplied with an electrolytic solution and then sealed, thereby producing the electric double-layer capacitor.
In the electric double-layer capacitor, the current collectors are not securely fixed to the electrodes. Therefore, when the electric double-layer capacitor vibrates, the current collectors and the electrodes tend to be relatively displaced out of intimate contact with each other, increasing the internal resistance of the electric double-layer capacitor.
One electric double-layer capacitor in which current collectors are attached to electrodes is disclosed in Japanese laid-open patent publication No. 64-9611. In the disclosed electric double-layer capacitor, an electrode is placed on a current collector in the form of an aluminum foil, and is embossed or drilled from the side of the aluminum foil, thus forming through holes in the electrode. The current collector has torn portions projecting along the inner peripheral edges of the through holes. Japanese laid-open patent publication No. 7-142091 reveals an electric double-layer capacitor including a current collector which comprises a metal sheet. The current collector is ripped to form holes with edge burrs extending from its face side to its back side. The current collector is pressed against an electrode of pelletized material which is penetrated by ends of the edge burrs.
According to Japanese laid-open patent publication No. 64-9611, portions of the current collector only project along the inner peripheral edge of the through holes. According to Japanese laid-open patent publication No. 7-142091, the edge burrs of the current collector only penetrate the electrode. Therefore, both the disclosed current collectors are disadvantageous in that when the electrode absorbs the electrolytic solution and is expanded thereby, the projecting portions or the penetrating edge burrs are liable to be pulled out, permitting the current collector to lose intimate contact with the electrode. Furthermore, according to Japanese laid-open patent publication No. 64-9611, since the through holes are defined in the electrode, the volume of the activated carbon of the electrode is less than would be if no through holes are defined in the electrode, resulting in a reduction in the electric capacitance of the electric double-layer capacitor.